1. Field of the Invention
This invention pertains generally to welding methods and apparatus, and more particularly, to such methods and apparatus that employ preheated filler material.
2. Description of the Prior Art
Nuclear reactor core barrel assemblies, because of their unusual size and weight, create unique welding problems in their construction that are not normally encountered in most manufacturing operations. The core barrel assemblies in pressurized water nuclear reactors are used to support the reactor internals and core components and are in turn supported by an upper flange on the internal ledge at the uppermost portion of the reactor pressure vessel. Generally, such reactor core barrel assemblies are constructed from four separate components: an upper, annular, circular flange; an upper, cylindrical, tubular barrel; a lower, cylindrical, tubular barrel; and a lower core support plate -- all of which are welded together. The assembly of the upper flange/upper core barrel section/lower core barrel section defines a generally tubular member, one end of which is substantially closed by the lower core support plate. The approximate height of a total representative assembly is in the order of 9.3 meters, with an inside diameter of approximately 3.7 meters. The general thickness of the upper and lower barrel sections amounts to a little over 5.08 centimeters. While the lower core support plate only occupies approximately a little over 50.8 centimeters of the total height of the core barrel assembly, it accounts for a substantial amount of the total weight, contributing approximately 27.3 metric tons to the entire assembly weight of approximately 72.6 metric tons.
The lower two circumferential weld joints which couple the the core support plate to the lower barrel portion and the lower barrel portion to the upper barrel are critically important in that high quality welds must be attained to maintain precise dimensional control of the barrel cylinders as they are welded. For example, the final design requires that: the longitudinal shrinkage between any two barrel cylinders or similar pieces be controlled within a 0.102 centimeter tolerance zone; the parallelism of the top flange relative to the core support be maintained within a 0.05 centimeter tolerance during longitudinal shrinkage; and the diametrical shrinkage be constant throughout the 360.degree. weld for each weld seam to minimize rotational distortion and to maintain coincidence of the x-y axes between the top flange and the core support within 0.050 centimeters.
Because of the tight design control stipulated, the core barrel welds are attained with the barrel cylinders in the vertical position, with welding accomplished in the horizontal plane. Conventional welding tolerances and a better weight distribution between the various components would permit a more standard welding practice of horizontally rolling the cylinders beneath the welding torch. However, it can be appreciated from the representative dimensions given above (although it should be understood that the dimensions will vary depending upon the size of the particular nuclear reactor under consideration) that such conventional techniques will most likely result in severe distortions in the alignment of the various components after the welds are complete. Such distortions are intolerable in the unique structure of a nuclear reactor.
To meet the aforementioned criteria in accordance with the prior art, welding has been accomplished by placing the barrel sections with their axes vertical, one barrel on the other. Manual welding techniques have been employed at various locations around the weld seam with the objective of producing an X-ray quality weld, yet control weld shrinkage and distortion. However, difficulty has been generally encountered in producing welds which meet both the quality and dimensional criteria because of the difficulty of simultaneously and uniformly distributing heat around each of the welding locations. Many repairs are normally necessary, and the time and cost expended are extensive. Local repairs to defective welds can result in further unacceptable distortions.
A recent innovation in the art of welding has provided for preheating the filler material prior to being deposited at the weld zone where it is fused with the workpiece by a separate welding torch. However, the quality of welds produced from this innovation has been found to vary extensively as a function of a number of the welding parameters, i.e. the amount of heat imparted to preheat the filler material and the speed of deposit. Reproducibility of quality welds obtained with this process has proved difficult because of a number of variables that have to be simultaneously controlled.
Accordingly, an improved method is desired that will produce high quality welds in large barrel cylinders and other components in a very precise, predictable and controlled manner. In addition, such a method must control vertical distortion and insure reproducibility.